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SubjectRe: What I suspect


On Wed, 8 Dec 1999, Richard Gooch wrote:
>
> 1a) the kernel exports a global page which has kernel data, such as the
> current time. Since the kernel exports it, it knows where it is

It knows the physical address. It doesn't know the linear address. Which
means that it can't contain code (or rather, the code would have to be PIC
for no good reason - nasty for performance).

> 1b) user-space can look for the known page and read values from it, if
> it wants

It's not about reading values. The time offsets are completely useless if
the CPU doesn't support rdtsc. We want to export CODE, not data.

> 2) MAP_INHERIT can be used by user-space to implement
> machine-specific code.

Sure. MAP_INHERIT can be used for general-purpose things. But MAP_INHERIT
is not going to happen. It has security issues that you ignore,a dn it has
performance issues for process creation (we can't just completely break
down and re-build the page tables, we have to be careful). End of story.

It's a neat idea, but as I've said before, it sucks in real life.

Linus


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